so the gammas are basicly high energy xrays, or hard xrays, a surface with high enough density should be capable of deflecting them, a multi layer system would probably eliminate all of it. xrays have enough trouble going through hardened steel.

Penetration of gamma rays depends on it's energy. Steel will stop 50,000 eV gammas or x-rays with only thin layers. A million eV gamma is a different story. Lead, tungsten or uranium would do better per thickness, unit of weitht, but the number of gammas to absorb where perhaps 1 part in 10,000 to 50,000 of the fusion energy is huge. In a 100 Megawatt output, that would be
~ 2,000 Watts of gamma energy. That is enough to quickly fry anybody near a non shielded machine. Without looking it up , I think a few second exposure may be lethal.

I calculated somewhere that for a reactor to be safe to work near for long periods, you'd want roughly a foot of lead shielding. These gammas are 4, 12, and 16 MeV, occurring due to gamma decay of excited ¹²C (excess energy of 16 MeV) once in every ~10,000 p-¹¹B reactions.

This is not a particularly well-documented reaction branch, though, and it's possible (if unlikely) that it won't turn out to be a problem...

I've been fiddling with data from Wikipedia on p11B fusion, which suggests that the side reaction

p+11B->12C+Gamma (16MeV) occurs with a 0.1 % branching probability.

Given this, I think a 10MW (Th) reactor would be a source of kilowatts of 16MeV gammas, and I agree with the last post that a foot of lead is about what is needed to stay healthy a couple of metres from such a thing.

However, there is no definitive reference, and looking at tables of excited states 12C* I can't see such a decay mentioned.

Does anyone have a definitive reference, or a suggestion where to search for such a thing?